System and method for performing an image level snapshot and for restoring partial volume data

ABSTRACT

The present invention relates to a method for performing an image level copy of an information store. The present invention comprises performing a snapshot of an information store that indexes the contents of the information store, retrieving data associated with the contents of the information store from a file allocation table, copying the contents of the information store to a storage device based on the snapshot, and associating the retrieved data with the copied contents to provide file system information for the copied contents.

Applicant(s) hereby claim the benefit of provisional patent applicationSer. Nos. 60/519,876 and 60/519,576, entitled “SYSTEM AND METHOD FORPERFORMING A SNAPSHOT AND FOR RESTORING DATA,” and “SYSTEM AND METHODFOR PERFORMING AN IMAGE LEVEL SNAPSHOT AND FOR RESTORING PARTIAL VOLUMEDATA,” respectively, each filed on Nov. 13, 2003, respectively. Theseapplications are incorporated by reference herein in their entirety.

COPYRIGHT NOTICE

A portion of the disclosure of this patent document contains materialwhich is subject to copyright protection. The copyright owner has noobjection to the facsimile reproduction by anyone of the patent documentor the patent disclosures, as it appears in the Patent and TrademarkOffice patent files or records, but otherwise reserves all copyrightrights whatsoever.

RELATED APPLICATIONS

This application is related to the following patents and pending patentapplications, each of which is hereby incorporated herein by referencein its entirety:

-   -   U.S. Pat. No. 6,418,478, entitled “PIPELINED HIGH SPEED DATA        TRANSFER MECHANISM,” issued Jul. 9, 2002;    -   application Ser. No. 09/610,738, entitled “MODULAR BACKUP AND        RETRIEVAL SYSTEM USED IN CONJUNCTION WITH A STORAGE AREA        NETWORK,” filed Jul. 6, 2000;    -   Application Ser. No. 09/744,268, entitled “LOGICAL VIEW AND        ACCESS TO PHYSICAL STORAGE IN MODULAR DATA AND STORAGE        MANAGEMENT SYSTEM,” filed Jan. 30, 2001;    -   application Ser. No. 60/409,183, entitled “DYNAMIC STORAGE        DEVICE POOLING IN A COMPUTER SYSTEM,” filed Sep. 9, 2002;    -   application Ser. No. 10/681,386 entitled “SYSTEM AND METHOD FOR        MANAGING STORED DATA,” filed Oct. 7, 2003; and    -   application Ser. No. 60/460,234, entitled “SYSTEM AND METHOD FOR        PERFORMING STORAGE OPERATIONS IN A COMPUTER NETWORK,” filed Apr.        3, 2003.

BACKGROUND OF THE INVENTION

The invention disclosed herein relates generally to a system and methodfor performing a snapshot and for restoring data. More particularly, thepresent invention relates to a system and method for performingsnapshots of an information store, which are stored across multiplestorage devices, and for restoring partial or full snapshots.

To obtain a more thorough understanding of the present invention, thefollowing discussion provides additional understanding regarding themanner is which magnetic media is used to store information. Usingtraditional techniques, copies of an information store are performedusing the operating system's file system. Copying is done by accessingthe operating system's (OS) file system for the information store to bebacked-up, such as the Windows NTFS file system. The file allocationsystem of the operating system typically uses a file allocation table tokeep track of the physical or logical clusters across which each file inthe information store is stored. Also called an allocation unit, acluster is a given number of disk sectors that are treated as a unit,each disk sector storing a number of bytes of data. This unit, thecluster, is the smallest unit of storage the operating system canmanage. For example, on a computer running Microsoft's Windows 95operating system, the OS uses the Windows FAT32 32-bit file allocationtable having a cluster size to 4K. The number of sectors is determinedwhen the disk is formatted by a formatting program, generally, but notnecessarily, when the OS is installed.

The operating system allocates disk space for a file only when needed.That is, the data space is not preallocated but allocated dynamically.The space is allocated one cluster at a time, where a cluster is a givennumber of consecutive disk sectors. The clusters for a file are chainedtogether, and kept track of, by entries in a file allocation table(FAT).

The clusters are arranged on the disk to minimize the disk headmovement. For example, all of the space on a track is allocated beforemoving on to the next track. This is accomplished by using thesequential sectors on the lowest-numbered cylinder of the lowestnumbered platter, then all sectors in the cylinder on the next platter,and so on, until all sectors on all platters of the cylinder are used.This is performed sequentially across the entire disk, for example, thenext sector to be used will be sector 1 on platter 0 of the nextcylinder.

For a hard (fixed) disk, FAT, sector, cluster, etc. size is determinedwhen a disk formatting program formats the disk, and are based on thesize of the partition. To locate all of the data that is associated witha particular file stored on a hard disk, the starting cluster of thefile is obtained from the directory entry, then the FAT is referenced tolocate the next cluster associated with the file. Essentially, the FATis a linked list of pointers to clusters on the disk, e.g., each 16-bitFAT entry for a file points to the next sequential cluster used for thatfile. The last entry for a file in the FAT has a number indicating thatno more clusters follow. This number can be from FFF8 to FFFF (base 16)inclusive.

FIG. 1 shows an example directory entry 2 of a Windows-formatted harddisk and accompanying FAT 20. The exemplary directory entry 2 consistsof 32 bytes of data. The name of the file and its extension are storedin the first eleven bytes 4 of the directory entry 2 and a fileattribute byte 6 is provided. By definition, ten bytes 8 are reservedfor future use and four bytes are provided to store time 10 and date 12information (two bytes each). Two cluster bytes 14 point to the firstcluster of sectors used to store the file information. The last fourbytes 18 of the directory entry 2 are used to store the size of thefile.

A sixteen-byte section of a FAT 20 is depicted. The first four bytes 21store system information. A two-byte pair, bytes four and five (16), arethe beginning bytes of the FAT 20 used to track file information. Thefirst cluster for data space on all disks is cluster “02.” Therefore,bytes four and five (16) are associated with the first cluster of disksectors “02” used to store file information. Bytes six and seven (22)are associated with cluster “03” . . . and bytes fourteen and fifteen(24) are associated with cluster “07.”

This example illustrates how sectors associated with a file referencedin a directory are located. The cluster information bytes 14 in thedirectory 2 point to cluster number “02.” The sectors in cluster “02”(not shown), contain the initial sector of data for the referenced file.Next, the FAT is referenced to see if additional clusters are used tostore the file information. FAT bytes four and five (16) were pointed toby the cluster information bytes 14, and the information stored in bytesfour and five (16) in the FAT 20 point to the next cluster used for thefile. Here, the next cluster is “05”. Accordingly, cluster “05” containsthe next sector of data for the referenced file. FAT bytes ten andeleven (26) contain an end-of-file flag, “FFFF,” indicating there are nomore clusters associated with the referenced file. All of theinformation comprising the referenced file, therefore, is contained inclusters “02” and “05” on the disk.

As with other applications running on the computer, a typical backupapplication provides a read request to the operating system, whichhandles interpretation of the information contained in the FAT andreading of each file for the backup application. A file system isprovided on the storage device that is used by the backup application towrite files that are copied to the device. Similarly, the recoveryportion of the backup application, or a separate recovery application,may read files from the storage device for recovery of the information.

Inherent problems and disadvantages have been discovered with currentlyavailable systems and methods for archiving data contained in aninformation store. One technique is to perform a full copy of the datacontained in the information store. Utilizing this technique results intwo separate copies of the information store, and the length of time ittakes to make this kind of copy is related to the amount of data copiedand the speed of the disk subsystem. For example, assuming a transferrate of 25 MB/sec, the approach will take one hour to copy 90 GB ofdata. These techniques, however, in addition to other disadvantages,require the applications on the information store to be quiesced duringthe copy routine. This places a significant burden on systemadministrators to complete copying and get critical systems back intothe production environment as, absent a high-speed data bus, the copyingmay consume a significant amount of time to complete.

Administrators typically keep multiple copies of a given informationstore. Unfortunately, this has the drawback of requiring n times theamount of space of the information store to maintain n copies, which canbe quite expensive to store, in addition to requiring complex and timeconsuming techniques for restoration of the copied data.

One currently available alternative is to perform snapshots of aninformation store. With current snapshot systems and methods,administrators create an incremental copy that is an exact point-in-timereplica of the source volume each time a snapshot is taken. A series ofsnapshot are stored locally on the information store from which it wastaken and track incremental changes to the data in the informationstore. Furthermore, changed data is written to a new location in theinformation store as tracked by the snapshot. With knowledge regardingthe change, as well as the changed data, the snapshot can be used to“roll back” changes to an information store to the point in time whenthe snapshot was taken. If there should be any logical corruption in theinformation store's data that went un-detected for a period of time,however, these incremental updates faithfully replicate that logicalcorruption to the data when copying. Additionally, other drawbacks areassociated with currently know snapshot techniques, including thesignificant drawback of preventing restoration from the snapshot in theevent that the information store fails, as both the snapshot and theinformation store become unavailable.

Systems and methods are needed, therefore, that overcome problemsassociated with currently known techniques for taking, maintaining andrestoring snapshots.

SUMMARY OF THE INVENTION

The present invention addresses, among other things, the problemsdiscussed above with copying up data using systems and methods known tothose of skill in the art. The invention provides systems and methodsfor performing n snapshots of an information store, without requiring ntimes the space of the information store, and storing those snapshots inmultiple destinations across a network.

One embodiment of the system of the present invention creates thesnapshots by taking a snapshot that indexes only clusters for files thatwere created or changed since the last snapshot. A snapshots, t_(n), isrestored by restoring the clusters from the snapshot t_(n). The clustersthat were not restored from snapshot t_(n) are restored from snapshott_(n-1), etc., until the remaining clusters are restored from the firstsnapshot, snapshot t₀.

In accordance with some aspects of the present invention, multiplesnapshots are kept on a storage device, without requiring n times thespace of the total volume of the information store. The system createssnapshots at various points in time that index only clusters for filesthat were created or changed since the last snapshot, and creates a copyof the data that has been changed or created. This allows users to keepseveral snapshots without requiring n times the space of the totalvolume of the information store.

In some embodiments, the system stores a map, which may be part of asnapshot, to track specific files and folders with their correspondingcopied clusters. The map created by reading data from the fileallocation table of the information store and associates files andfolders with the clusters stored in the snapshots. In this way, eventhough the snapshot was performed at the cluster level, individual orgroups of files and/or folders may be restored without unnecessarilyrestoring the entire information store.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is illustrated in the figures of the accompanying drawingswhich are meant to be exemplary and not limiting, in which likereferences are intended to refer to like or corresponding parts, and inwhich:

FIG. 1 is an example directory entry for a file in a prior art FAT of aWindows-formatted hard disk;

FIG. 2 is a block diagram illustrating a network architecture forperforming snapshot operations according to one embodiment of thepresent invention;

FIG. 3 is a flow diagram illustrating a method for creating a snapshotaccording to one embodiment of the present invention;

FIG. 4 is a block diagram illustrating the relationship between a mapand a snapshot according to one embodiment of the present invention;

FIG. 5 is a flow diagram illustrating a method for restoring a snapshotaccording to one embodiment of the present invention; and

FIG. 6 is a flow diagram illustrating a method for restoring specificfiles or folders from a snapshot according to one embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to FIGS. 2 through 6, embodiments of the presentinvention are shown. FIG. 2 presents a block diagram illustrating thecomponents of a system for performing storage and restoration operationson electronic data in a computer network according to one embodiment ofthe invention. It should be understood that the invention is not limitedto networked environments, and may also be implemented on a stand-alonecomputer or electronic device.

As shown, the system of FIG. 2 includes a storage manager 100, includinga volume replication table 102 and a storage manager index cache 120,and one or more of the following: a client 85, an information store 90,a data agent 95, a media agent 105, a media agent index cache 110, and astorage device 115. One exemplary embodiment of the present system isthe CommVault QuNetix three-tier system available from CommVaultSystems, Inc. of Oceanport, N.J., further described in U.S. patentapplication Ser. No. 09/610,738 and hereby incorporated by reference inits entirety.

A data agent 95 is a software module that is generally responsible forretrieving data from an information store 90 for copies, snapshots,archiving, migration, and recovery of data stored in an informationstore 90 or other memory location, e.g., hard disc drive. Each clientcomputer 85 preferably has at least one data agent 95 and the system cansupport many client computers 85. The data agent 95 provides aninterface to an information store 90 to execute copies, snapshots,archiving, migration, recovery and other storage operations on data inconjunction with one or more media agents 105. According to oneembodiment, each client 85 runs a number of data agents 95, wherein eachdata agent is configured to interface with data generated by or from aspecific application, e.g., a first data agent to interface withMicrosoft Exchange data and a second data agent to interface with Oracledatabase data. As is explained in greater detail herein, a data agent 95is in communication with one or more media agents 105 to effect thedistributed storage of snapshots on one or more storage devices 115 thatare remote from the information store that is the source of the snapshot90.

The storage manager 100 is a software module or application thatcoordinates and controls other components comprising the system, e.g.,data and media agents, 95 and 105, respectively. The storage manager 100communicates with data 95 and media 105 agents to control and managesnapshot creation, migration, recovery and other storage operations.According to one embodiment, the storage manger 100 maintains data in astorage manager index cache 120 that instructs a given data agent 95 towork in conjunction with a specific media agent 105 to store snapshotson one or more storage devices 115.

The storage manager 100 maintains a storage manager index cache 120.Data in the storage manager index cache 120, which the storage manager100 collects from data agents 95, media agents 105, user and otherapplications, is used to indicate, track and associate: logicalrelationships and associations between components of the system, userpreferences, management tasks, and other data that is useful to thesystem. For example, the storage manager index cache 120 may containdata that tracks logical associations between media agents 105 andstorage devices 115. The storage manager index cache 120 may alsocontain data that tracks the status of storage operations to beperformed, storage patterns such as media use, storage space growth,network bandwidth, service level agreement (“SLA”) compliance levels,data protection levels, storage policy information, storage criteriaassociated with user preferences, data retention criteria, storageoperation preferences, and other storage-related information.

A media agent 105 is a software module that transfers data inconjunction with one or more data agents 95, as directed by the storagemanager 100, between an information store 90 and one or more storagedevices 115, such as a tape library, a magnetic media storage device, anoptical media storage device, or other storage device. The media agent105 communicates with and controls the one or more storage devices 115.According to one embodiment, the media agent 105 may communicate withthe storage device 115 via a local bus, such as a SCSI adaptor.Alternatively, the storage device 115 may communicate with the dataagent 105 via a Storage Area Network (“SAN”). Other types ofcommunication techniques, protocols and media are contemplated asfalling within the scope of the invention.

The media agent 105 receives snapshots, preferably with the changed datathat is tracked by the snapshot, from one or more data agents 95 anddetermines one or more storage devices 115 to which it should write thesnapshot. According to one embodiment, the media agent 105 appliesload-balancing algorithms to select a storage device 115 to which itwrites the snapshot. Alternatively, the storage manager 100 instructsthe media agent 105 as to which storage device 115 the snapshot shouldbe written. In this manner, snapshots from a given information store 90may be written to one or more storage devices 115, ensuring data isavailable for restoration purposes in the event that the informationstore fails. Either the media agent or the storage manager 100 recordsthe storage device on which the snapshot is written in a replicationvolume table 102, thereby allowing the snapshot to be located whenrequired for restoring the information store 90.

A media agent 105 maintains a media agent index cache 110 that storesindex data the system generates during snapshot, migration, and restoreoperations. For example, storage operations for Microsoft Exchange datagenerate application specific index data regarding the substantiveExchange data. Similarly, other applications may be capable ofgenerating application specific data during a copy or snapshot. Thisdata is generally described as metadata, and may be stored in the mediaagent index cache 110. The media agent index cache 110 may track datathat includes, for example, information regarding the location of storeddata on a given volume. The media agent index cache 110 may also trackdata that includes, but is not limited to, file names, sizes, creationdates, formats, application types, and other file-related information,information regarding one or more clients associated stored data,information regarding one or more storage policies, storage criteria,storage preferences, compression information, retention-relatedinformation, encryption related information, and stream relatedinformation. Index data provides the system with an efficient mechanismfor locating user files during storage operations such as copying,performing snapshots and recovery.

This index data is preferably stored with the snapshot that is backed upto the storage device 115, although it is not required, and the mediaagent 105 that controls the storage operation may also write anadditional copy of the index data to its media agent index cache 110.The data in the media agent index cache 110 is thus readily available tothe system for use in storage operations and other activities withouthaving to be first retrieved from the storage device 115.

In order to track the location of snapshots, the system uses a databasetable or similar data structure, referred to herein as a replicationvolume table 102. The replication volume table 102, among otheradvantages, facilitates the tracking of multiple snapshots acrossmultiple storage devices 115. For example, the system might, as directedby a policy or a user, store a first snapshot t₀on first storage deviceA, such as a tape drive or library, and then store subsequent snapshotscontaining only the changed culster(s), t_(n), on a second storagedevice B, such as an optical drive or library. Alternatively,instructions may be stored within system components, e.g., a storagemanger 100 or media agent 105, directing the storage devices 115 used tostore snapshots. Information regarding the storage device 115 to whichthe snapshot is written, as well as other information regarding thesnapshot generally, is written to the replication volume table 102. Anexemplary structure according to one embodiment is as follows:

( id serial, // PRIMARY KEY FOR THIS TABLE PointInTime integer, //CreationTime integer, // Timestamp of RV creation ModifyTime integer, //Timestamp of last RV update CurrentState integer, // Current state of RVCurrentRole integer, // Current role of RV PrimaryVolumeId integer, //FOREIGN KEY FOR SNRVolume TABLE PhysicalVolumeId integer, // FOREIGN KEYFOR SNRVolume TABLE ReplicationPolicyId integer, // FOREIGN KEY FORReplicationPolicy TABLE RVScratchVolumeId integer, // FOREIGN KEY FORRVScratchVolume table Flags integer, JobId LONGLONG, SnapVolumeIdinteger, // FOREIGN KEY FOR SNRVolume TABLE }

In the exemplary replication volume table, id is a unique identificationnumber assigned by the system to the snapshot; PointInTime representsthe date and time that the snapshot was created; CreationTime representsthe date and time that the snapshot was completed; ModifyTime is therecorded date and time of the snapshot taken prior to the currentsnapshot; CurrentState is an identifier used to indicate a currentstatus of the snapshot (e.g. pending, completed, unfinished, etc.);PrimaryVolumeId is the identifier for the information store 90 fromwhich the snapshot is being made; PhysicalVolumeId is a hardwareidentifier for the information store 90; RVScratchVolumeId is anidentifier for a scratch volume, which in some embodiments may be usedto buffer additional memory as known to those of skill in the art; Flagscontains a 32 bit word for various settings such as whether a snapshothas been taken previously, etc.; JobId stores the identifier for the jobas assigned by a storage management module; and the SnapVolumeId pointsto the physical destination storage device 115 to which the snapshot iswritten.

As each snapshot indexes an information store at a given point in time,a mechanism must be provided that allows the snapshots taken of aninformation store to be chronologically related so that they areproperly used for restoring an information store 90. According to thereplication volume table 102, the CurrentRole integer may store a valuefor the relative position of a given snapshot in hierarchy of snapshotstaken from a given information store 90 (e.g. first (t0), second (t1),t2, t3, etc.)

In some embodiments, components of the system may reside on and beexecuted by a single computer. According to this embodiment, a dataagent 95, media agent 105 and storage manager 100 are located at theclient computer 85 to coordinate and direct local copying, archiving,migration, and retrieval application functions among one or more storagedevices 115 that are remote or distinct from the information store 90.This embodiment is further described in U.S. patent application Ser. No.09/610,738.

One embodiment of a method for using the system of the present inventionto perform snapshots is illustrated in the flow diagram of FIG. 3. Whenthe system is initialized, or at other times as directed by a user orrules, e.g., policies or other instructions, the storage manager directsthe data agent to perform an initial full snapshot of the data stored inthe information store, e.g., indexing the location of all data in theinformation store, in conjunction with one or more media agents. Thesystem copies all of the data on the information store with the initialsnapshot to a storage device, step 300.

Advantageously, the snapshot and data copied from the information storemay be written to a storage device that is remote or different from theinformation store, step 302, e.g., local data from a given informationstore is written to a storage device attached to a network. Theselection of a destination storage device for the snapshot may beaccomplished using one or more techniques known to those of skill in theart. For example, a fixed mapping may be provided indicating a storagedevice for which all snapshots and copied or changed data should bewritten. Alternatively, an algorithm may be implemented to dynamicallyselect a storage device from among a number of storage devices availableon a network. For example, a storage manager may select a media agent tohandle the transfer of the snapshot and copied data to a specificstorage device based on criteria such as available bandwidth, otherscheduled storage operations, media availability, storage policies,storage preferences, or other consider considerations. The snapshot,preferably along with the data from the information store, is written tothe selected destination storage device, step 304. According to certainembodiments, the snapshot contains information regarding the files andfolders that are tracked by the snapshot. Alternatively, the informationregarding the files and folders that are indexed by the snapshot, e.g.,file system information, are stored on the storage device.

One embodiment of a snapshot used to track clusters read from theinformation store to clusters in a snapshot, as well as to map file andfolder names corresponding to the snapshot clusters, is illustrated inFIG. 4. It should be noted that clusters are but one level ofgranularity that may be indexed by a snapshot, e.g., blocks, extents,etc. During the scan, the data agent creates a snapshot 350 and writesdata, e.g., new or changed data, to a storage device 115. According tothe present embodiment, the snapshot is illustrated as a flat file datastructure, although those of skill in the art will recognize that thesnapshot may be embodied in a number of disparate types of datastructures.

The snapshot 350 is used to associate the original cluster numbers froman information store with clusters on a storage device, which in thepresent embodiment is a magnetic tape. It should be appreciated by thoseof skill in the art that the present invention is not limited tomagnetic tape, and that the systems and methods described herein may beapplicable to using snapshots with other storage technologies, e.g.,storing disk geometry data to identify the location of a cluster on astorage device, such as a hard disk drive.

The tape offsets 356 for the clusters 372 in the snapshot 370 are mappedto original disk cluster information 352. File and folder names 354 maybe scanned from the information store's FAT and also mapped to the tapeoffsets 356. A file part column 358 in the snapshot tracks the clusters372 for each file and folder where each file and folder contains anentry for the first cluster 372. For files or folders that are stored inmore than one cluster, sometimes not in contiguous clusters, the offsettable entry for each further cluster is numbered consecutively 358.

In order to identify the files and folders represented by the storedclusters 372, e.g., changed data, in the snapshot 370, the map mayexclude data from columns relating to the original disc clusters 352 andlast snapshot 360. In order to keep track of changed verses unchangedclusters, however, the original disk cluster information 352 is storedin the map 350. Other information may also be stored in the map 350,such as timestamps for last edit and creation dates of the files.

For each snapshot, even though only clusters that have been changed orcreated since a previous snapshot are tracked in a given snapshot afterthe initial snapshot t₀, the snapshot may be provided with the data fromall previous snapshots to provide the latest snapshot with folder andfile information such that an index of the entire information store ismaintained concurrently each snapshot. Alternatively, this may bebypassed in favor of creating a snapshot that indexes all data at agiven point in time in the information store and copying only changeddata.

Entries from each snapshot 350 may also contain a last-snapshot field360 that holds an identifier for the last snapshot containing thecluster indexed by the entry at the time the current snapshot wascreated. According to an alternative embodiment, e.g., for snapshotsthat do not store the information from the information store's FAT, thesnapshot only tracks clusters stored in the information store with theclusters indexed by the snapshot. For those embodiments, the snapshot350 contains neither file and folder information 345 nor file partinformation 358.

Returning to FIG. 3, once the first full snapshot to has been taken,step 300, the storage manager may implement a rule, policy, or similarset of instructions that require snapshots to be taken at certain timeintervals. Accordingly, at each time interval where a subsequentsnapshot t_(n) is taken, the data agent works in conjunction with one ormore of the media agents to perform and store snapshot and accompanyingdata that changed since the subsequent snapshot, t_(n-1), loop 306.

For each snapshot, t_(n), that is taken of the information store, acomparison is performed such that only the clusters which have changedor been created since the last snapshot, t_(n-1), was taken of thatvolume are stored, step 310. For example, in some embodiments the dataagent employs a block filter or similar construct known to those ofskill in the art to compare snapshot t_(n) with t_(n-1) and therebydetect changed clusters on an information store. Alternatively, the dataagent may use other techniques know in the art, such as Copy on Write(“COW”), to identify changed data on an information store. If a givencluster in the information store has changed since the last snapshot inwhich the cluster appears, or if the cluster from the information storewas created subsequent to the last snapshot, then the cluster is readfrom information store and stored with the new snapshot being written tothe storage device, step 314.

A determination is made regarding the given storage device to which thesnapshot and changed data (which may also include newly created data) isto be written, step 316. Techniques such as those described inconjunction with storage of the initial snapshot, steps 302 and 304, mayalso be employed regarding storage of subsequent snapshots.Advantageously, the initial snapshot and any subsequent snapshot maywritten to any storage device available in the network. Furthermore,there is no limitation to the combination of devices used to store thesnapshots for a given information store. For example, an initialsnapshot may be written to storage device A, a second and thirdsnapshots may be written to storage device B, and a fourth snapshot maybe written to storage device C. Regardless of the storage device that isselected, step 316, the replication volume table is updated to reflectthe location, step 318, allowing snapshots to be located when a userrequests to restore the information store from which the snapshots weretaken.

System administrators use stored snapshots, in conjunction with thechanged data that the snapshot indexes or tracks, to recover lost orcorrupted information. FIG. 5 presents a flow diagram illustrating oneembodiment of a method for restoring an information store from one ormore snapshots. If the user or a system process wants to restore aninformation store from one or more snapshots, an interface is presentedto restore the snapshot, step 400. The interface may be, for example, agraphical user interface (“GUI”), and Application Programming Interface(“API”) or other interface known to those of skill in the art. Thestorage manager scans the replication volume table to identify availablesnapshots for presentation in a menu that allows selection of anavailable snapshot, step 402.

When the user selects a snapshot, the storage manager performs a queryof the replication volume table to identify all previous snapshots foran information store from which the selected snapshot was taken, step404. This may be accomplished by performing a search on the replicationvolume table for all snapshots with the same PrimaryVolumeId orPhysicalVolumeId. Starting with the selected snapshot, for each snapshotin the query result, loop 406, the storage manager directs a given mediaagent, in conjunction with a given data agent, to read and restore allclusters of changed data not already restored from clusters indexed by aprior snapshot, e.g., the latest version of each cluster, step 408.According to one embodiment, this is accomplished by restoring theclusters indexed by each of the snapshots in the query result, startingwith the original snapshot, and overwriting clusters indexed by theoriginal snapshot with changed clusters indexed by subsequent snapshotsup to the snapshot representing the point in time selected by the useror system process. As an alternative, the last snapshot field of theselected snapshot may be utilized to determine the snapshots that shouldbe utilized in the restore operation. The latest version of eachcluster, starting with those indexed by the selected snapshot, is thenrestored, step 408.

As discussed above, embodiments of the invention are contemplatedwherein FAT information of the information store is stored inconjunction with a given snapshot, e.g. the file and folder informationcorresponding to the clusters of changed data indexed by a givensnapshot. Accordingly, the storage manager may allow the user to selectindividual files and/or folders to be selected for restoration from asnapshot. With reference to FIG. 6, a flow diagram is presentedillustrating one embodiment of a method for restoring individual filesand/or folders indexed by a snapshot.

When the user desires to restore the information store to a given pointin time, the user interface allows the user to view the files andfolders indexed by a snapshot representing the point in time as if theuser were viewing a folder structure on a storage device, step 500. Thestorage manager retrieves the file and folder information for changeddata that is indexed by one or more snapshots for display. Once one ormore files and/or folders are selected, step 502, the storage managerselects those snapshots that index the given version of the files and/orfolders using the replication volume table, step 502. Each snapshotindexing data for the one or more files to be restored are openedserially, loop 506. The changed data for the selected files and foldersthat are indexed by the snapshots are restored from clusters indexed byeach snapshot, step 508, but not overwriting clusters indexed by priorsnapshots.

While the invention has been described and illustrated in connectionwith preferred embodiments, many variations and modifications as will beevident to those skilled in this art may be made without departing fromthe spirit and scope of the invention, and the invention is thus not tobe limited to the precise details of methodology or construction setforth above as such variations and modification are intended to beincluded within the scope of the invention.

1. A method in a computing system of creating snapshots for aninformation store, the method comprising: performing a first snapshot ofdata in an information store at a first storage device at a first time;selecting a first storage medium at a second storage device for storageof the data associated with the first snapshot, wherein the secondstorage device is separate from the first storage device; storing thedata associated with the first snapshot in the first storage medium;tracking changes to the data associated with the first snapshot, thechanges occuring between the first time and a second time occuring afterthe first time; performing at least a second snapshot of the data in theinformation store at the second time; selecting a second storage mediumfor storage of the tracked changes to the data associated with the firstsnapshot, wherein the second storage medium is different from theinformation store; storing, in the second storage medium, the trackedchanges to the data associated with the first snapshot, wherein thestoring of the tracked changes comprises copying the tracked changes tothe second storage medium using information associated with the secondsnapshot to identify a location in the information store of the trackedchanges to the data associated with the first snapshot; and tracking alocation of the data associated with the first snapshot and the locationin the information store of the tracked changes to the data associatedwith the first snapshot.
 2. The method of claim 1, wherein trackingchanges to the data associated with the first snapshot comprisestracking changes to the data associated with the first snapshot using ablock filter to identify changes to the data associated with the firstsnapshot and then storing data identifying the changes to the dataassociated with the first snapshot in a data structure.
 3. The method ofclaim 1, wherein tracking changes to the data associated with the firstsnapshot comprises tracking changes to the data associated with thefirst snapshot using a block filter to identify changes to the dataassociated with the first snapshot.
 4. The method of claim 1, whereinthe first storage medium and the second storage medium comprise the samestorage medium.